Shape memory materials are characterized as those materials that may undergo reversible transformation between two distinct solid phases. The distinct solid phases may be referred to as a martensitic phase and an austenitic phase. Such transformation may in general be induced by exposure to an external stimulus such as, e.g., a change in temperature or applied mechanical stress. In general, shape memory materials dissipate energy during transformation between martensitic and austenitic phases. This energy dissipation is due, in general, to the creation and motion of internal material interfaces during the phase transformations, and the amount of energy that is dissipated is directly related to the transformation stress and strain.
The most widely employed shape memory materials are metals, and in particular metal alloys. Shape memory alloys (SMAs) are well-known for their ability to transform between martensitic and austenitic phases. Preexisting SMA structures are characterized by relatively low transformation stresses and correspondingly low energy dissipation capabilities. Meanwhile, some ceramic materials have been shown to be capable of exhibiting reversible martensitic transformation with high stresses, offering the prospect of improved energy dissipation over that of preexisting SMAs and the ability to particularly address applications in, e.g., actuation, energy harvesting, and mechanical energy damping.
However, in general, because the martensitic transformation and its associated shape change generally leads to substantial internal stresses, ceramics, which are in general brittle materials, have a tendency to crack during such transformation. As a result, ceramics may in general exhibit only very small shape memory strains and commensurately low energy dissipation levels, and tend to fracture or crack during such processes. Thus, although ceramic materials could in principle exhibit shape memory and superelastic properties with useful transformation shape recovery, such properties are not achievable thus far due to the brittle nature of such ceramic materials.